Process for producing lithium secondary battery

ABSTRACT

A process for producing a lithium secondary battery employs a charging method where a positive electrode upon charging has a maximum achieved potential of 4.3 V (vs. Li/Li+) or lower. The process includes charging the lithium secondary battery to reach at least a region with relatively flat fluctuation of potential appearing in a positive electrode potential region exceeding 4.3 V (vs. Li/Li + ) and 4.8V (vs. Li/Li + ) or lower. The lithium secondary battery includes an active material having a solid solution of a lithium transition metal composite oxide having an α-NaFeO 2  type crystal structure. The composition ratio of Li, Co, Ni, and Mn contained in the solid solution satisfies Li 1−1/3x Co 1−x−y Ni y/2 Mn 2x/3+y/2 (x+y≦1, 0≦y and 1−x−y=z).

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application of Ser. No. 12/734,579 filed on May11, 2010.

TECHNICAL FIELD

The present invention relates to an active material for a lithiumsecondary battery and a lithium secondary battery using the same.

BACKGROUND ART

Conventionally, for a lithium secondary battery, LiCoO₂ is mainly usedas a positive active material. However, a lithium secondary batteryusing LiCoO₂ as a positive active material has a discharge capacity ofabout 120 to 130 mAh/g and is also inferior in thermal stability in thebattery inside in a charging state.

Therefore, materials obtained by forming solid solutions of LiCoO₂ withother compounds are known as the active material for a lithium secondarybattery. That is, as an active material for a lithium secondary battery,Li[Co_(1−2x)Ni_(x)Mn_(x)]O₂ (0<x≦½), which is a solid solution having anα-NaFeO₂ type crystal structure shown on a ternary phase diagram ofthree components, LiCoO₂, LiNiO₂, and LiMnO₂, was disclosed in 2001. Alithium secondary battery using one example of the above-mentioned solidsolutions, LiNi_(1/2)Mn_(1/2)O₂ or LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂ has adischarge capacity of 150 to 180 mAh/g and is thus more excellent thanthat using LiCoO₂ and also more excellent in thermal stability in thebattery inside in a charging state than that using LiCoO₂.

However, an active material for a lithium secondary battery with afurther higher discharge capacity has been required.

Patent Documents 1 to 4 disclose compounds obtained by adding Fe toLi[Li_(1/3)Mn_(2/3)]O₂ as active materials for a lithium secondarybattery. Patent Documents 5 to 8 disclose compounds obtained by addingFe and Ni to Li[Li_(1/3)Mn_(2/3)]O₂ as active materials for a lithiumsecondary battery.

However, although being characterized in that economical iron is used asa raw material, lithium secondary batteries using the materials of theinventions disclosed in Patent Documents 1 to 8 have high polarizationas compared with those using conventional positive active materials andare not also excellent in discharge capacity.

Patent Documents 9 and 10 disclose LiNiO₂—Li[Li_(1/3)Mn_(2/3)]O₂ typesolid solutions as active materials for a lithium secondary battery.

However, the active materials for a lithium secondary battery disclosedin Patent Documents 9 and 10 have a problem that their synthesis needsto be carried out in oxygen and synthesis in air is difficult since theelectron state of Ni is Ni³⁺. As described above, also in terms of theindustrial handling easiness, an active material for a lithium secondarybattery in which Ni is present in form of Ni²⁺ is desired. Further,since merely one electron reaction of Ni³⁺→Ni⁴⁺ is employed in thismaterial, improvement of the discharge capacity of a lithium secondarybattery cannot be expected.

Patent Documents 11 and 12 discloseLiNi_(1/2)Mn_(1/2)O₂—Li[Li_(1/3)Mn_(2/3)]O₂ type solid solutions and thelike as active materials for a lithium secondary battery.

However, the discharge capacities of lithium secondary batteries usingthe materials disclosed in Patent Documents 11 and 12 are far fromimprovement; the discharge capacities are inferior to those in the caseof using LiNi_(1/2)Mn_(1/2)O₂ alone.

Patent Documents 13 and 14 disclose materials obtained by allowingLi[Li_(1/3)Mn_(2/3)]O₂ to be present on the particle surfaces of LiMeO₂(Me: Co, Ni) as active materials for a lithium secondary battery.

However, the techniques disclosed in Patent Documents 1 to 14 and thetechniques disclosed in Patent Documents 15 to 18 described below allfail to improve the discharge capacity, which is an object of thepresent invention.

Disclosing a concept of employing solid solutions of three components,Li[Ni_(1/2)Mn_(1/2)]O₂, Li[Li_(1/3)Mn_(2/3)]O₂, and LiCoO₂ as a basicstructure, Patent Documents 15 and 16 contain descriptions as follows:

“The present invention provides a layered lithium transition metalcomposite oxide supposed to form a solid solution of

Li[Ni_(1/2)Mn_(1/2)]O₂ at a ratio of (1−3×)(1−y)

Li[Li_(1/3)Mn_(2/3)]O₂ at a ratio of 3x(1−y), and

LiCoO₂ at a ratio of y,

and having a layered structure, that is, a basic structure of[Li]^((3a))[(Li_(x)Ni_((1−3x)/2)Mn_((1+x)/2))_((1−y))Co_(y)]^((3b))O₂ .. . (II), wherein (3a) and (3b) respectively represent different metalsites in the layered R(−3)m structure”, “However, the important point ofthe present invention is that z mol of Li is added excessively to thecomposition represented by the formula (II) and a solid solutionrepresented as

[Li]^((3a))[Li_(z/(2+z)){(Li_(x)Ni_((1−3x)/2)Mn_((1+x)/2))_((1−y)Co_(y)}_(2/(2+z))]^((3b))O₂  (I)

(wherein, 0.01≦x≦0.15; 0≦y≦0.35; 0.02(1−y)(1−3x)≦z≦0.15(1−y)(1−3x)); and(3a) and (3b) respectively represent different metal sites in thelayered R(−3)m structure.” (paragraphs 0018 and 0019). However, alsowith reference to Comparative Examples, merely those having excessamounts of Li beyond the amounts obtained spontaneously in the case ofassuming such solid solutions are concretely described and there is nodescription that the discharge capacity can be improved by specifyingthe ratios of those three components in the composition range in whichthe Li amount is made not to be intentionally in excess.

Patent Document 17 discloses the composition formula:(Li[Ni_((x−y))Li_((1/3−2x/3))Mn_((2/3−x/3−y))Co_(2y)]O₂ (0<x≦0.5; 0≦y≦⅙;x>y) in claim 1.

The composition formula disclosed in claim 1 of Patent Document 17partially overlaps the composition range of the present invention as abroader concept; however there is no description implying the technicalidea of employing the solid solution of three components ofLi[Ni_(1/2)Mn_(1/2)]O₂, Li[Li_(1/3)Mn_(2/3)]O₂, and LiCoO₂ and the rangeshowing the above-mentioned composition formula widely includescompositions other than those of the solid solution of three componentsof Li[Ni_(1/2)Mn_(1/2)]O₂, Li[Li_(1/3)Mn_(2/3)]O₂, and LiCoO₂.

Patent Document 18 discloses the composition formula: (Li[Ni_((x−y))Li_((1/3−2x/3))Mn_((2/3−x/3−y))Co_(2y)]O₂ (wherein x is more than 0 and0.5 or less; y is 0 or more and ⅙ or less, and x>y) in claim 2.

The composition formula disclosed in claim 2 of Patent Document 18partially overlaps the composition range of the present invention as abroader concept; however as Examples, merely “a compound represented bythe composition formula Li[Ni_(0.5)Mn_(0.5)]O₂” and “a compoundrepresented by the composition formula Li[Ni_(0.4)Mn_(0.4)Cu_(0.2)]O₂”are concretely disclosed and they are completely out of the compositionrange of the present invention. Further, there is no descriptionimplying the technical idea of employing the solid solution of threecomponents of Li[Ni_(1/2)Mn_(1/2)]O₂, Li[Li_(1/3)Mn_(2/3)]O₂, andLiCoO₂.

Patent Document 19 discloses a method for synthesizingLi[Co_(1−2x)Ni_(x)Mn_(x)]O₂ having an α-NaFeO₂ type crystal structure byproducing a hydroxide of transition metals (Co, Ni, Mn) by acoprecipitation method, mixing the hydroxide with a lithium compound,and calcining the mixture.

Patent Document 1: JP-A No. 2002-068748

Patent Document 2: JP-A No. 2002-121026

Patent Document 3: Japanese Patent No. 03500424

Patent Document 4: JP-A No. 2005-089279

Patent Document 5: JP-A No. 2006-036620

Patent Document 6: JP-A No. 2003-048718

Patent Document 7: JP-A No. 2006-036621

Patent Document 8: Japanese Patent No. 03940788

Patent Document 9: JP-A No. 09-055211

Patent Document 10: Japanese Patent No. 03539518

Patent Document 11: JP-A No. 2004-158443

Patent Document 12: Japanese Patent No. 03946687

Patent Document 13: JP-A No. H08-17935

Patent Document 14: Japanese Patent No. 03258841

Patent Document 15: JP-A No. 2006-253119

Patent Document 16: JP-A No. 2007-220475

Patent Document 17: JP-A No. 2004-006267

Patent Document 18: JP-A No. 2004-152753

Patent Document 19: International Publication No. 02/086993

Non-patent Document 1 discloses preparation and electrochemicalproperties of a solid solution of LiCoO₂—LiNi_(0.5)Mn_(0.5)O₂—Li₂MnO₃with a high Mn amount and concretely discloses

0.36LiCoO₂-0.2LiNi_(1/2)Mn_(1/2)O₂-.0.44Li₂MnO₃,0.27LiCoO₂-0.2LiNi_(1/2)Mn_(1/2)O₂-0.53Li₂MnO₃,0.18LiCoO₂-0.2LiNi_(1/2)Mn_(1/2)O₂-0.62Li₂MnO₃, and

0.09LiCoO₂-0.2LiNi_(1/2)Mn_(1/2)O₂-0.71Li₂MnO₃; however in a case wherethe Li content is as high as 1.4 to 1.5, the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and (104)plane measured by X-ray diffractometry is about 1.4 (see FIG. 2), whichis not 1.56 or higher, and therefore, these solid solutions areapparently different from the active material of the present invention.Further, with respect to the production method, only calcining at 750 to950° C. after decomposition of respective acetic acid salts at 400° C.by a spray drying method is described but no method of employing acoprecipitation method is described. Moreover, although the dischargecapacity is increased to be 200 mAh/g or higher in a potential region of3.0 to 4.6 V, an increase of the discharge capacity in a potentialregion of 4.3 V or lower is not indicated.

Non-patent Document 2 discloses that with respect toLi[Li_(0.182)Ni_(0.182)Co_(0.091)Mn_(0.545)]O₃, that is, a layeredmaterial of0.545Li[Li_(1/3)Mn_(2/3)]O₂-0.364LiNi_(1/2)Mn_(1/2)O₂-0.091LiCoO₂, thedischarge capacity is 200 mAh/g or higher in a potential region of 4.6 Vto 2.0 V of an initial period and about 160 mAh/g in a potential regionof 4.3 V to 2.0 V after cycles in 4.6 V to 2.0 V and therefore, thislayered material does not have a high discharge capacity in thepotential region of 4.3 V or lower. Further, the layered material isproduced by producing a slurry of respective acetic acid salts, dryingthe slurry at 120° C. and calcining the dried product at 900° C. andthus is not produced by a coprecipitation method and the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and (104)plane measured by X-ray diffractometry is about 1, which is not 1.56 orhigher, and therefore, the material is apparently different from theactive material of the present invention.

Non-patent Document 3 discloses that as a positive active material of alithium battery, 0.7Li₂MnO₃.0.3LiMn_(0.33)Ni_(0.33)Co_(0.33)O₂ and0.5Li₂MnO₃.0.5LiMn_(0.33)Ni_(0.33)Cu_(0.33)O₂ are shown and with respectto the former, the discharge capacity is 261 mAh/g at 4.8 V charge at50° C. and 200 mAh/g at 4.6 V charge at 50° C., but improvement of thedischarge capacity in a potential region of 4.3 V or lower is notdescribed. Further, the above-mentioned positive active materials areproduced by mixing a coprecipitated hydroxide of Co, Ni, and Mn withLiOH, pre-sintering the mixture at 300 or 500° C., and calcining thepre-sintered product at 800 to 1000° C. and the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and (104)plane measured by X-ray diffractometry is about 1, which is not 1.56 orhigher, and therefore, the materials are apparently different from theactive material of the present invention.

Non-patent Document 4 discloses Li[Li_(1/5)Ni_(1/10)Co_(1/5)Mn_(1/2)]O₂,that is, a solid solution having a layered crystal structure of0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.2LiNi_(1/2)Mn_(1/2)O₂-0.2LiCoO₂, and theintensity ratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003)plane and (104) plane measured by X-ray diffractometry is about 1.4 (seeFIG. 3), which is not 1.56 or higher, and therefore, the solid solutionis apparently different from the active material of the presentinvention. Further, with respect to the production method, merely asol-gel method using respective acetic acid salts is described, andproduction by using a coprecipitation method is not described. Moreover,although the discharge capacity is described to be 229 mAh/g at 4.5 V;improvement of the discharge capacity in a potential region of 4.3 V orlower is not described.

Non-patent Document 5 discloses an active material of(1−2x)LiNi_(1/2)Mn_(1/2)O₂.xLi[Li_(1/3)Mn_(2/3)]O₂.xLiCoO₂ (0≦x≦0.5),and 0.2LiNi_(1/2)Mn_(1/2)O₂.0.4Li[Li_(1/3)Mn_(2/3)]O₂.0.4LiCoO₂,0.5Li[Li_(1/3)Mn_(2/3)]O₂-0.5LiCoO₂ or the like satisfying thecomposition formula has composition close to that of the presentinvention but not in the range of the composition of the presentinvention. Further, with respect to the production method, merely asolid-phase method using respective acetic acid salts is described, andproduction using a coprecipitation method is not described. Moreover,since the discharge capacity is about 190 mAh/g at 4.6 V (x=0.4), thedischarge capacity in a potential region of 4.3 V or lower is not sohigh.

Non-patent Document 6 discloses a positive active material ofLiNi_(0.20)Li_(0.20)Mn_(0.60)O₂, that is,0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.4LiNi_(1/2)Mn_(1/2)O₂, and the intensityratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and(104) plane measured by X-ray diffractometry is about 1.7 (see FIG. 7)and the intensity of the diffraction peak of the (104) plane becomeshigher than the intensity of the diffraction peak of the (003) planeafter discharge, and therefore, this positive active material isapparently different from the active material of the present invention.Further, with respect to the production method, merely a method ofcalcining powders obtained by heat decomposition of respective aceticacid salts or nitric acid salts is described, and production by using acoprecipitation method is not described. Moreover, the dischargecapacity is described to be 288 mAh/g at 4.8 V charge in the initialperiod and 220 mAh/g after 20 cycles; however improvement of thedischarge capacity in a potential region of 4.3 V or lower is notdescribed.

Non-patent Document 7 discloses a positive active material of a layeredstructure of (1−x−y)LiNi_(1/2)Mn_(1/2)O₂.xLi[Li_(1/3)Mn_(2/3)]O₂.yLiCoO₂(0≦x=y≦0.3 and x+y=0.5) and since the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎between the diffraction peaks on (003) plane and (104) plane measured byX-ray diffractometry of0.5LiNi_(1/2)Mn_(1/2)O₂.0.5Li[Li_(1/3)Mn_(2/3)]O₂ satisfying thecomposition formula is about 1.4, which is not 1.56 or higher, andtherefore, the positive active material is apparently different from theactive material of the present invention. Further, with respect to theproduction method, merely a solid-phase method using respective aceticacid salts is described, and production using a coprecipitation methodis not described. Moreover, since the discharge capacity is about 180mAh/g at 4.6 V, the discharge capacity in a potential region of 4.3 V orlower is not so high.

Non-patent Document 8 discloses a solid solution with a layeredstructure of 0.5Li(Ni_(0.5)Mn_(0.5))O₂-0.5Li(Li_(1/3)Mn_(2/3))O₂, and inQ24, which is a solid solution of a lithium transition metal compositeoxide having an α-NaFeO₂ type crystal structure, the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and (104)plane measured by X-ray diffractometry is about 1.2, which is not 1.56or higher, and therefore, this solid solution is apparently differentfrom the active material of the present invention. In S24 and VS24, theintensity ratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003)plane and (104) plane measured by X-ray diffractometry is 1.56 orhigher; however many peaks of impurities are observed and they are notspecified as the solid solution of a lithium transition metal compositeoxide having an α-NaFeO₂ type crystal structure. Further, with respectto the production method, merely a method of calcining precursors fromrespective acetic acid salts is described, and production using acoprecipitation method is not described. Moreover, although thedischarge capacity of Q24 is about 210 mAh/g at 4.6 V charge,improvement of the discharge capacity in a potential region of 4.3 V orlower is not described. S24 and VS24 are those having small dischargecapacities.

Non-patent Document 9 discloses electrochemical properties of a solidsolution of Li(Li_((1−x)/3)Co_(x)Mn_((2−2x)/3)O₂) (0≦x≦1), and inLi(Li_(0.7/3)Co_(0.3)Mn_(1.4/3)O₂) satisfying the composition formula,that is, 0.7Li[Li_(1/3)Mn_(2/3)]O₂-0.3LiCoO₂, andLi(Li_(0.6/3)Co_(0.4)Mn_(1.2/3)O₂), that is,0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.4LiCoO₂, the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎between the diffraction peaks on (003) plane and (104) plane measured byX-ray diffractometry is about 1.3, which is not 1.56 or higher, andtherefore, this solid solution is apparently different from the activematerial of the present invention. Further, with respect to theproduction method, merely a method of calcining precursors fromrespective acetic acid salts is described, and production using acoprecipitation method is not described. Moreover, although thedischarge capacity is described to be about 250 mAh/g at 4.6 V charge,improvement of the discharge capacity in a potential region of 4.3 V orlower is not described.

Non-patent Document 10 discloses synthesis, structure, andelectrochemical behaviors of Li[Ni_(x)Li_(1/3−2x/3)Mn_(2/3−x/3)]O₂ andwith respect to the production method, production using acoprecipitation method is described; however inLi[Ni_(0.25)Li_(1/6)Mn_(7/12)]O₂ satisfying the composition formula,that is, a solid solution of0.5Li[Li_(1/3)Mn_(2/3)]O₂-0.5LiNi_(1/2)Mn_(1/2)O₂ and the like, theintensity ratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003)plane and (104) plane measured by X-ray diffractometry is about 1, whichis not 1.56 or higher, and therefore, the solid solution is apparentlydifferent from the active material of the present invention. Further,although the discharge capacity is described to be about 220 mAh/g at4.8 V charge, improvement of the discharge capacity in a potentialregion of 4.3 V or lower is not described (based on the observation ofthe charge-discharge curve, about 150 mAh/g in terms of 4.3 V).

Non-patent Document 11 discloses synthesis and electrochemicalproperties of a compound of Li[Co_(x)Li_((1/3−x/3))Mn_((2/3−2x/3))]O₂,and in a compound of Li[Co_(0.33)Li_(0.67/3)Mn_(1.34/3)]O₂ satisfyingthe composition formula, that is, a compound of0.67Li[Li_(1/3)Mn_(2/3)]O₂-0.33LiCoO₂, the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎between the diffraction peaks on (003) plane and (104) plane measured byX-ray diffractometry is about 1.4, which is not 1.56 or higher, andtherefore, this compound is apparently different from the activematerial of the present invention. Further, with respect to theproduction method, merely a method of calcining powders obtained by heatdecomposition of respective acetic acid salts or nitric acid salts isdescribed, and production by using a coprecipitation method is notdescribed. Moreover, although the discharge capacity is described to beabout 200 mAh/g at 4.6 V charge, improvement of the discharge capacityin a potential region of 4.3 V or lower is not described (based on theobservation of the charge-discharge curve, about 150 to 160 mAh/g interms of 4.3 V).

Non-patent Document 12 discloses the results of X-ray diffractometry ofa positive active material of Li(Li_(0.2)Ni_(0.2)Mn_(0.6))O₂ for alithium secondary battery, that is, a positive active material of0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.4LiNi_(1/2)Mn_(1/2)O₂ and that the intensityratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and(104) plane is about 1.6 and 1.7 after discharge is described; howeverit is about 1.2, which is not 1.56 or higher, after synthesis and beforedischarge and therefore, the positive active material is apparentlydifferent from the active material of the present invention. Further,with respect to the production method, merely a sol-gel method usingrespective acetic acid salts is described, and production using acoprecipitation method is not described. Moreover, although thedischarge capacity is about 200 mAh/g in a potential region of 2.0 to4.6 V, the discharge capacity is about 110 mAh/g in a potential regionof 2.0 to 4.3 V after 4.6 V charge and therefore, the discharge capacityin the potential region of 4.3 V or lower is not so high.

Non-patent Document 13 discloses a nanocrystal ofLi[Li_(0.2)Ni_(0.2)Mn_(0.6)]O₂, that is 0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.4LiNi_(1/2)Mn_(1/2)O₂, and the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ between thediffraction peaks on (003) plane and (104) plane measured by X-raydiffractometry is about 1.3, which is not 1.56 or higher, and therefore,this nanocrystal is apparently different from the active material of thepresent invention. Further, with respect to the production method,merely a method of calcining powders obtained by heat decomposition ofrespective acetic acid salts or nitric acid salts is described, andproduction by using a coprecipitation method is not described. Moreover,although the discharge capacity is described to be about 210 mAh/g at4.8 V charge, improvement of the discharge capacity in a potentialregion of 4.3 V or lower is not described.

Non-patent Document 14 discloses preparation and electrochemicalbehaviors of a solid solution ofLiCoO₂—Li₂MnO₃(Li[Li_((x/3))Co_((1−x))Mn_((2x/3))]O₂), and inLi[Li_(0.2)Co_(0.4)Mn_(0.4)]O₂ satisfying the composition formula, thatis, a solid solution of 0.6Li[Li_(1/3)Mn_(2/3)]O₂-0.4LiCoO₂ andLi[Li_(0.23)Co_(0.31)Mn_(0.46)]O₂, that is, a solid solution of0.69Li[Li_(1/3)Mn_(2/3)]O₂-0.31LiCoO₂, the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎between the diffraction peaks on (003) plane and (104) plane measured byX-ray diffractometry (see FIG. 2) is about 2.3 and 1.9, respectively,before charge-discharge, which is 1.56 or higher; however at the end ofcharge with a charge capacity of 160 mAh or higher, the intensity ratiois considerably lowered (1.4 to 1.7 for the former solid solution, seeFIG. 10) and the intensity ratio at the end of discharge in the case ofdischarge of the active material (solid solution) with the considerablylowered intensity ratio is not made clear and therefore, the activematerials cannot be said to be the same as the active material of thepresent invention. Further, with respect to the production method,merely a method of decomposing respective acetic acid salts at 400° C.by a spray drying method and thereafter, calcining at 750 to 950° C. isdescribed, and production using a coprecipitation method is notdescribed. Moreover, the discharge capacity is about 100 mAh/g at 4.5 Vcharge and thus it is not so high.

Non-patent Document 15 discloses a positive active material of a layeredstructure of 0.6LiNi_(0.5)Mn_(0.5)O₂.xLi₂MnO₃.yLiCoO₂ (x+y=0.4), and in0.6LiNi_(0.5)Mn_(0.5)O₂.0.4Li₂MnO₃ satisfying the composition formula,the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003)plane and (104) plane measured by X-ray diffractometry is about 1.4,which is not 1.56 or higher, and therefore, this positive activematerial is apparently different from the active material of the presentinvention. Further, with respect to the production method, merely amethod of calcining powders obtained by heat decomposition of respectiveacetic acid salts is described, and production using a coprecipitationmethod is not described. Moreover, although the discharge capacity isdescribed to be about 210 mAh/g at 4.6 V charge, improvement of thedischarge capacity in a potential region of 4.3 V or lower is notdescribed (about 150 mAh/g in terms of 4.3V).

Non-patent Document 16 discloses a positive active material ofLi[Li_(0.15)Ni_(0.275)Mn_(0.575)]O₂ for a lithium secondary battery,that is, a positive active material of0.45Li[Li_(1/3)Mn_(2/3)]O₂-0.55LiNi_(1/2)Mn_(1/2)O₂ and the intensityratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and(104) plane measured by X-ray diffractometry is about 1, which is not1.56 or higher, and therefore, this positive active material isapparently different from the active material of the present invention.Further, with respect to the production method, merely a sol-gel methodusing respective acetic acid salts is described, and production using acoprecipitation method is not described. Moreover, although thedischarge capacity is described to be about 180 mAh/g at 4.6 V charge,improvement of the discharge capacity in a potential region of 4.3 V orlower is not described (about 140 mAh/g in terms of 4.3V).

Non-patent Document 17 discloses synthesis and electrochemicalproperties of Li[Li_((1−2x)/3)Ni_(x)Mn_((2−x)/3)]O₂ as a positive activematerial for a lithium secondary battery, and inLi[Li_(0.15)Ni_(0.275)Mn_(0.575)]O₂ satisfying the composition formula,that is, a positive active material of0.45Li[Li_(1/3)Mn_(2/3)]O₂-0.55LiNi_(1/2)Mn_(1/2)O₂, the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and (104)plane measured by X-ray diffractometry is about 1, which is not 1.56 orhigher, and therefore, the positive active material is apparentlydifferent from the active material of the present invention. Further,with respect to the production method, merely a sol-gel method usingrespective acetic acid salts is described, and production using acoprecipitation method is not described. Moreover, although thedischarge capacity is described to be about 190 mAh/g at 4.6 V charge,improvement of the discharge capacity in a potential region of 4.3 V orlower is not described (about 140 mAh/g in terms of 4.3V).

Non-patent Document 1: Electrochimica Acta 51 (2006)5581-5586

Non-patent Document 2: Electrochemistry Communications 7 (2005)1318-1322

Non-patent Document 3: Electrochemistry Communications 9 (2007)787-795

Non-patent Document 4: Journal of Power Sources 146 (2005)281-286

Non-patent Document 5: Journal of Power Sources 146 (2005)598-601

Non-patent Document 6: Solid State Ionics 176 (2005)1035-1042

Non-patent Document 7: Journal of The Electrochemical Society,152(1)A171-A178 (2005)

Non-patent Document 8: Journal of Power Sources 124 (2003)533-537

Non-patent Document 9: Electrochemistry Communications 9 (2007)103-108

Non-patent Document 10: Journal of The Electrochemical Society,149(6)A777-A791 (2002)

Non-patent Document 11: Journal of The Electrochemical Society,151(5)A720-A727 (2004)

Non-patent Document 12: Electrochemical and Solid-State Letters, 6(9)A183-A186 (2003)

Non-patent Document 13: Electrochemical and Solid-State Letters, 6(8)A166-A169 (2003)

Non-patent Document 14: Journal of Power Sources 159 (2006)1353-1359

Non-patent Document 15: Materials Letters 58 (2004)3197-3200

Non-patent Document 16: Journal of Materials Chemistry, 2003, 13,319-322

Non-patent Document 17: Journal of Power Sources 112 (2002)634-638

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In consideration of the above-mentioned problems, it is an object of thepresent invention to provide an active material for a lithium secondarybattery with a high discharge capacity, particularly capable ofincreasing the discharge capacity in a potential region of 4.3 V orlower and a method for producing the same. It is also an object of theinvention to provide a lithium secondary battery with a high dischargecapacity and a method for producing the same.

Means for Solving the Problems

The present invention will be described, with reference to the technicalidea. However, the functional mechanism includes assumption and whetherthe assumption is correct or not does not limit the present invention atall. The invention can be carried out in various manners withoutdeparting from the spirit and main characteristics of the invention.Accordingly, the invention should not to be construed as being limitedsince the foregoing embodiments or experimental examples are onlyillustrations in any ways. The scope of the invention is shown by theclaims and is not bound to the specification. Modifications andalterations belonging to the equivalent scope of the claims are withinthe invention.

In the case of using conventionally known LiMnO₂ as an active materialfor a lithium secondary battery, since Jahn-Teller distortion attributedto the redox reaction of Mn⁴±/Mn³⁺ in the charge-discharge process iscaused, a stable discharge capacity cannot be obtained.

Further, in a conventionally known material ofLi[Co_(1−2x)Ni_(x)Mn_(x)]O₂ (0<x≦½), which is a solid solution having anα-NaFeO₂ type crystal structure shown on a ternary phase diagram ofthree components, LiCoO₂, LiNiO₂, and LiMnO₂, regarding the valences ofthe transition metal elements at the time of synthesis, the valences ofnot only Co and Ni but also Mn are fluctuated along withcharge-discharge. However, only in a particular case where Ni and Mn arepresent at the same ratio, it is experimentally known that electronstate of Ni²⁺, Mn⁴⁺ or Co³⁺ is possible and only in this case, since thevalence of Mn is not changed as being tetravalence even ifelectrochemical reduction and oxidation (insertion/extraction oflithium) is carried out, it is supposed that a desirable reversibleproperty can be obtained. In addition, in this case, along with theelectrochemical oxidation, the valence of Ni is changed from divalenceto trivalence and further to tetravalence and the valence of Co ischanged from trivalence to tetravalence. Herein, a particular case whereNi and Mn are present at the same ratio is corresponding to a point onthe straight line shown in the ternary phase diagram of threecomponents, LiCoO₂, LiNiO₂, and LiMnO₂. However, out of the straightline, electron state of Ni²⁺, Mn⁴⁺ or Co³⁺ becomes impossible and itresults in failure of obtaining an excellent discharge capacity andcharge-discharge cycle performance.

Materials supposed to have the valences of the respective metal elementsas Li⁺, Co³⁺, Ni²⁺, and Mn⁴⁺ are also discovered partially in PatentDocuments 15 to 18.

However, as described above, even if with reference to the descriptionsof Patent Documents 15 to 18, it is impossible to obtain any materialhaving a discharge capacity as a secondary battery higher than that of aconventional material.

There is monoclinic Li[Li_(1/3)Mn_(2/3)]O₂ as a representative layeredstructure containing Li⁺ and Mn⁴⁺. As described in the above-mentionedPatent Documents 1 to 14, various compounds based on thisLi[Li_(1/3)Mn_(2/3)]O₂ have been investigated so far. However, it isknown that Li[Li_(1/3)Mn_(2/3)]O₂ is scarcely possible to obtain acharge-discharge capacity if it is used alone. It is supposedly becauseno redox reaction of Mn⁴⁺→Mn⁵⁺ is generated in the stable range of acommon organic electrolyte solution.

Focusing on that the valence of Mn in the above-mentionedLi[Li_(1/3)Mn_(2/3)]O₂ is tetravalence, the present inventors have madeinvestigations of forming a solid solution of Li[Li_(1/3)Mn_(2/3)]O₂with other compounds. Consequently, the inventors have supposed thateven if electrochemical reduction and oxidation (charge-discharge) iscarried out, the valence of Mn is not changed from tetravalence but thevalences of transition metal elements constituting other compoundsforming a solid solution with Li[Li_(1/3)Mn_(2/3)]O₂ can be changed andaccordingly a high discharge capacity can be obtained and also a stablecharge-discharge cycle performance can be obtained.

Further, the present inventors have made investigations of a ternarysolid solution of LiCoO₂—LiNi_(1/2)Mn_(1/2)O₂—Li[Li_(1/3)Mn_(2/3)]O₂ byadding LiCoO₂ to the binary phase diagram. Since LiCoO₂ is excellent inthe initial charge-discharge efficiency and also excellent in the highrate charge-discharge property, the inventors have supposed that theseproperties could be utilized advantageously.

The ternary solid solution is expressed as a ternary phase diagram asshown in FIG. 1. All compounds on this matrix are present by taking theforms of Co³⁺, Ni²⁺, and Mn⁴⁺. That is, in the above-mentionedLiCoO₂—LiNiO₂—LiMnO₂ system, as shown in FIG. 2, Ni and Mn can bepresent as Ni²⁺ and Mn⁴⁺ only on the line where Ni and Mn are present atthe same ratio, whereas in the ternary solid solution ofLiCoO₂—LiNi_(1/2)Mn_(1/2)O₂—Li[Li_(1/3)Mn_(2/3)]O₂, Co, Ni, and Mn canbe present in the forms of Co³⁺, Ni²⁺, and Mn⁴⁺ in any point within thephase.

Accordingly, the above-mentioned ternary solid solution, which is a baseof the present invention, can be represented asx{Li[Li_(1/3)Mn_(2/3)]O₂}.y{LiNi_(1/2)Mn_(1/2)O₂}.(z=1−x−y){LiCoO₂}.This is deformed to lead the formula,Li_(1+(1/3)x)Co_(1−x−y)Ni_((1/2)y)Mn_((2/3)x+(1/2)y)O₂. Herein, inaccordance with the definition, 0≦x, 0≦y, and x+y≦1.

The present inventors have found that in a case where x is particularlyin a range satisfying ⅓<x in the ternary solid solution, a lithiumsecondary battery using the material as an active material shows aconsiderably higher discharge capacity than that in the case of using aconventional material and simultaneously have found that the battery isalso excellent in the cycle stability and thus have previously filed anapplication as Japanese Patent Application No. 2007-293777. Further, theinventors have found that in a case where x is particularly in a rangesatisfying ⅓<x≦⅔ in the ternary solid solution, a lithium secondarybattery using the material as an active material shows a considerablyhigher discharge capacity than that in the case of using a conventionalmaterial and simultaneously have found that the battery is alsoexcellent in the cycle stability and thus have previously filed anapplication as Japanese Patent Application No. 2007-330259.

Now, the inventors have found that in a case where x is in a specifiedrange satisfying ⅓<x and a specified property is satisfied in theternary solid solution, a lithium secondary battery using the materialas an active material has a considerably high discharge capacity,particularly a very high discharge capacity in a potential region of 4.3V or lower.

As being understood from the composition formula, one characteristic ofthe active material composition of the present invention is that the Licontent is high as compared with that of a conventional active material.In view of merely this point, the active material composition of theinvention cannot be expressed by plotting on the composition diagram ofFIG. 2 describing a conventional technique. Further, the compositiondiagram of FIG. 2 cannot express the case of a+b+c=1 in the compositionformula, Li_(q)CoaNi_(b)Mn_(c)O_(d), likewise the material of theinvention.

Herein, the present invention provides an active material for a lithiumsecondary battery including a solid solution of a lithium transitionmetal composite oxide having an α-NaFeO₂-type crystal structure, inwhich the composition ratio of Li, Co, Ni, and Mn contained in the solidsolution satisfies Li_(1+(1/3)x)Co_(1−x−y)Ni_((1/2)y)Mn_((2/3)x+(1/2)y)(x+y≦1, 0≦y and 1−x−y=z); in anLi[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) type ternaryphase diagram, (x, y, z) is represented by values in a range present onor within a line of a heptagon (ABCDEFG) defined by the vertexes; pointA(0.45, 0.55, 0), point B(0.63, 0.37, 0), point C(0.7, 0.25, 0.05),point D(0.67, 0.18, 0.15), point E(0.75, 0, 0.25), point F(0.55, 0,0.45), and point G(0.45, 0.2, 0.35); and the intensity ratio between thediffraction peaks on (003) plane and (104) plane measured by X-raydiffractometry before charge-discharge is I₍₀₀₃₎/I₍₁₀₄₎≧1.56 and at theend of discharge is I₍₀₀₃₎/I₍₁₀₄₎>1.

Herein, “before charge-discharge” in the present invention means untilthe time when first electrochemical electric communication is carriedout after synthesis of the active material.

Further, “at the end of discharge” means after discharge of 160 mAh/g orhigher (after discharge of 177 mAh/g or higher in Examples). Concretely,as shown in Examples, charging to 4.3 V (vs. Li/Li+) is carried out andthen constant current discharge at 0.1 ItA current is carried out andthe time point when the termination voltage becomes 2.0 V is defined asthe end of discharge.

In general, in a case where a lithium transition metal composite oxidehaving an α-NaFeO₂ type crystal structure is synthesized through acalcining step, it is known that the composition of a compound to beobtained actually is slightly fluctuated as compared with thecomposition calculated from the composition ratio of loaded rawmaterials. The present invention can be carried out without departingfrom the technical idea or main characteristics of the invention andneedless to say, it should not be construed such that the fact that thecomposition obtained by synthesis is not strictly identical with theabove-mentioned composition formula does not belong to the scope of theinvention. Particularly, with respect to the Li amount, it is known thatevaporation occurs easily in the calcining step. Further, thecoefficient of oxygen atom tends to be fluctuated in accordance with thesynthesis conditions or the like. In the composition formula of claim 1of the invention, the coefficient of oxygen atom is not defined. Thecoefficient of O ofLi[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) in theternary phase diagram is not limited strictly to 2 but includesdeficiency.

The active material of the invention may contain other elements otherthan Li, Co, Ni, Mn, and O and also, in a case where the active materialcontains other elements other than Li, Co, Ni, Mn, and O if Li, Co, Ni,and Mn are given from the elements constituting the solid solution, thecomposition ratio satisfies the definition of the invention and theeffect of the invention is exerted, those containing other elements arealso within the technical scope of the invention. Examples of the otherelements other than Li, Co, Ni, Mn, and O may be transition metals otherthan Co, Ni, and Mn.

The active material for a lithium secondary battery of the presentinvention is capable of charge-discharge at around 4.5 V (vs. Li/Li⁺) ofthe positive electrode potential. However, depending on the type of anonaqueous electrolyte to be used, if the positive electrode potentialis too high at the time of charging, there is a fear that the nonaqueouselectrolyte is oxidized and decomposed and the battery performance islowered. Consequently, it is sometimes required to obtain a lithiumsecondary battery having a sufficient discharge capacity even if acharging method of adjusting the maximum achieved potential of thepositive electrode upon charging to 4.3 V (vs. Li/Li⁺) or lower isemployed at the time of use. In a case where an active material for alithium secondary battery in which (x, y, z) is within theabove-mentioned range, and the intensity ratio between the diffractionpeaks on (003) plane and (104) plane measured by X-ray diffractometrysatisfies the above-mentioned condition is used, even if a chargingmethod of adjusting the maximum achieved potential of the positiveelectrode upon charging to 4.3 V (vs. Li/Li⁺) or lower is employed atthe time of use, it is made possible to obtain a discharge electricquantity of 177 mAh/g or higher (almost all 180 mAh/g or higher), whichexceeds the capacity of a conventional positive active material.

Accordingly, the invention provides an active material for a lithiumsecondary battery including a solid solution of a lithium transitionmetal composite oxide having an α-NaFeO₂-type crystal structure, inwhich the composition ratio of Li, Co, Ni, and Mn contained in the solidsolution satisfies Li_(1+(1/3)x)Co_(1−x−y)Ni_((1/2)y)Mn_((2/3)x+(1/2)y)(x+y≦1, 0≦y, and 1−x−y=z); in anLi[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) type ternaryphase diagram, (x, y, z) is represented by values in a range present onor within a line of a heptagon (ABCDEFG) defined by the vertexes; pointA(0.45, 0.55, 0), point B(0.63, 0.37, 0), point C(0.7, 0.25, 0.05),point D(0.67, 0.18, 0.15), point E(0.75, 0, 0.25), point F(0.55, 0,0.45), and point G(0.45, 0.2, 0.35); and the intensity ratio between thediffraction peaks on (003) plane and (104) plane measured by X-raydiffractometry at the end of discharge is I₍₀₀₃₎/I₍₁₀₄₎>1 and thedischargeable electric quantity in a potential region of 4.3 V (vs.Li/Li⁺) or lower is 177 mAh/g or higher.

With respect to the active material for a lithium secondary battery, theabove-mentioned (x, y, z) is preferably represented by values in a rangepresent on or within a line of a tetragon (HIJK) defined by thevertexes; point H(0.6, 0.4, 0), point I(0.67, 0.13, 0.2), point J(0.7,0, 0.3), and point K(0.55, 0.05, 0.4).

In the case of being within the range, even if a charging method ofadjusting the maximum achieved potential of the positive electrode atthe time of charging to 4.3 V (vs. Li/Li⁺) or lower is employed, it ismade possible to obtain a discharge electric quantity of 198 mAh/g orhigher (almost all 200 mAh/g or higher), which significantly exceeds thecapacity of a conventional positive active material.

Further, the present invention provides a lithium secondary batteryincluding such an active material for a lithium secondary battery.

Herein, those containing merely a mixture of a LiCoO₂ powder, aLiNi_(1/2)Mn_(1/2)O₂ powder, and a Li[Li_(1/3)Mn_(2/3)]O₂ powder is notincluded in the “solid solution” contained in the active material for alithium secondary battery of the present invention. Since simple mixtureof these three materials have respectively different peak positionscorresponding to the lattice constants and observed in the case ofmeasuring by X-ray diffractometry, when X-ray diffractometry is carriedout, diffraction patterns corresponding to the respective simple mixturecan be obtained. However, the solid solution of a lithium transitionmetal composite oxide having an α-NaFeO₂ type crystal structure of theinvention forms solid solution of at least a portion ofLi[Li_(1/3)Mn_(2/3)]O₂ with LiCoO₂ and/or LiNi_(1/2)Mn_(1/2)O₂. Even ifthe above-mentioned (x, y, z) is in the above-mentioned range, in a casewhere Li[Li_(1/3)Mn_(2/3)]O₂ does not at all form a solid solution withLiCoO₂ and/or LiNi_(1/2)Mn_(1/2)O₂, the effect of the invention to givea lithium battery with a high discharge capacity cannot be exerted.

Furthermore, the active material for a lithium secondary battery of thepresent invention is an active material present in a range satisfyingx>⅓ and has a diffraction peak observed near 2θ=20 to 30° in X-raydiffractometry using CuKα radiation for the monoclinicLi[Li_(1/3)Mn_(2/3)]O₂. It is supposed to be a super-lattice lineobserved in a case where Li⁺ and Mn⁴⁺ are arranged in a regular order.In the invention, those particularly excellent in the discharge capacityhave about 4 to 7% intensity of the monoclinic type diffraction peakobserved near 210 to the diffraction peak intensity of the hexagonal(003) plane, which is a main peak. The intensity of the monoclinic typediffraction peak observed near 210 is increased in proportion to theincrease of the ratio of Li[Li_(1/3)Mn_(2/3)]O₂ in the solid solution.As a result, with respect to an active material with the intensity ofthe monoclinic type diffraction peak observed near 210 exceeding 7% tothe diffraction peak intensity of the hexagonal (003) plane, which is amain peak, it becomes contrarily difficult to obtain a sufficientdischarge capacity.

Herein, in the present invention, the solid solution of a lithiumtransition metal composite oxide is characterized in that a diffractionpeak observed for the monoclinic Li[Li_(1/3)Mn_(2/3)]O₂ is observed near20 to 300 in X-ray diffractometry using CuKα radiation.

Moreover, the present inventors have found that an active material for alithium secondary battery capable of particularly giving a lithiumsecondary battery with a high discharge capacity can be reliablysynthesized in the case of producing a precursor by coprecipitation of ahydroxide containing Co, Ni, and Mn in a solvent when a solid solutionof a lithium transition metal composite oxide is obtained through thesteps of mixing the precursor containing the transition metal elementsand a lithium compound and then calcining the mixture. The inventorssuppose that it is relevant to the fact that the distribution of thetransition metals (Co, Ni, Mn) in the precursor is uniformly carried outby a coprecipitation method of the transition metal hydroxide as theprecursor. In addition, the description of Patent Document 19 is a goodreference with respect to a preferable method for producing such aprecursor.

Herein, the present invention provide a method for producing an activematerial for a lithium secondary battery, in which the solid solution ofa lithium transition metal composite oxide is produced through the stepsof producing a precursor by coprecipitation of a hydroxide containingCo, Ni, and Mn in a solvent, mixing the precursor and a lithium compoundand calcining the mixture.

In order to produce a lithium secondary battery capable of giving asufficient discharge capacity by using the active material for a lithiumsecondary battery of the present invention and even if employing acharging method of adjusting the maximum achieved potential of thepositive electrode upon charging to 4.3 V (vs. Li/Li⁺) or lower at thetime of use, it is important to provide a charging step in considerationof the characteristic behavior of the active material for a lithiumsecondary battery of the invention in the production method of thelithium secondary battery. That is, if constant current charging iscontinued by using the active material for a lithium secondary batteryof the invention as a positive electrode, a region with relatively flatpotential fluctuation is observed for a relatively long period in apositive electrode potential of 4.3 V to 4.8 V. FIG. 9 shows comparisonof the positive electrode potential behaviors when charging is firstcarried out for the respective positive electrodes using the activematerials for a lithium secondary battery of Example 6 (AT17) andComparative Example 4 (AT11). The curve shown as “1st charge” in thedrawing corresponds to this. As being observed in FIG. 9A (Example 6), aregion with relatively flat potential fluctuation is observed for arelatively long period at a potential of around 4.45 V from a momentwhen the charge electricity exceeds 100 mAh/g in the first charging. Onthe other hand, in FIG. 9B (Comparative Example 4), such a flat regionis scarcely observed. The charging conditions used herein are a constantcurrent and constant voltage charge of 0.1 ItA current and 4.5 V (vs.Li/Li⁺) voltage (positive electrode potential) and even if the chargingvoltage is set to be further higher, the region with relatively flatpotential fluctuation for a relatively long period is scarcely observedin the case of using a material with an x value of ⅓ or lower. On theother hand, in the case of a material with an x value exceeding ⅔, theregion becomes short even if the region with relatively flat potentialfluctuation is observed. Further, this behavior cannot be observed for aconventional Li[Co_(1−2x)Ni_(x)Mn_(x)]O₂ (0≦x≦½) material. This behavioris characteristic for the active material for a lithium secondarybattery of the invention.

Herein, the present invention provides a method for producing thelithium secondary battery by employing a charging method where thepositive electrode upon charging has a maximum achieved potential of 4.3V (vs. Li/Li⁺) or lower, in which the method includes the step ofcharging to reach at least a region with relatively flat potentialappearing in a positive electrode potential region of exceeding 4.3 V(vs. Li/Li⁺) and 4.8 V (vs. Li/Li⁺) or lower.

Herein, it is required to carry out the charging in the initialcharge-discharge step before completion of a battery to the extent ofreaching the flat potential region. Since the flat potential regioncontinues relatively long (e.g., 100 mAh/g or higher), it is preferablethat the charging continues so as to be through the step as much aspossible. Herein, in a case where the terminal of the flat potentialregion due to a potential increase or the like is observed, it may beregarded as the charging termination condition, or in a case whereconstant current and constant voltage charging is employed to allow theelectric current value to be attenuated to a set value, it may beregarded as the charging termination condition.

EFFECTS OF THE INVENTION

Accordingly, the present invention can provide an active material for alithium secondary battery with a high discharge capacity, particularlycapable of increasing the discharge capacity in a potential region of4.3 V or lower and can also provide a lithium secondary battery with ahigh discharge capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: A diagram showing the technical idea and the technical scope ofthe present invention.

FIG. 2: A diagram for explaining the technical idea of a conventionaltechnique.

FIG. 3: A view showing the crystal structure of an active materialhaving a composition formula:Li_(1+(1/3)x)Co_(1−x−y)Ni_((1/2)y)Mn_((2/3)x+(1/2)y) (x+y≦1 and 0≦y).

FIG. 4A: An X-ray diffraction pattern of an active material assumed tobe LiNiO₂ (simulation result close to the measurement result of thepresent invention).

FIG. 4B: An X-ray diffraction pattern of an active material assumed tobe (Li_(0.8)Ni_(0.2))[Ni_(0.8)Li_(0.2)]O₂ (simulation result close tothe description of a document).

FIG. 5: X-ray diffraction patterns of conventional active materials madeof LiNi_(0.20)Li_(0.20)Mn_(0.60)O₂ and LiCu_(0.20)Li_(0.27)Mn_(0.53)O₂before charge-discharge, after charge, and after discharge.

FIG. 6: An X-ray diffraction pattern of the active material of Example 1(AT06).

FIG. 7: A view showing the XAFS measurement results of active materialsof Examples 1 to 4 and Comparative Example 41.

FIG. 8: An X-ray diffraction pattern of the active material ofComparative Example 3 (AT09).

FIG. 9: A view showing the potential behaviors at the time of initialcharge-discharge carried out in the lithium secondary battery productionmethod using the active materials of Example 6 (AT17) and ComparativeExample 4 (AT11).

FIG. 10: A view showing the potential behaviors of lithium secondarybatteries using the active materials of Example 6, Comparative Example4, and Comparative Example 42.

FIG. 11: X-ray diffraction patterns of the active material of Example 7(AT18) before charge-discharge (synthesized sample), after charge, andafter discharge.

FIG. 12: X-ray diffraction patterns of the active material of Example 16(AT33) before charge-discharge (synthesized sample), after charge, andafter discharge.

FIG. 13: An Li[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z)type ternary phase diagram in which the discharge capacity values ofactive materials of Examples 1 to 44 and Comparative Examples 1 to 40are plotted.

BEST MODE FOR CARRYING OUT THE INVENTION

As described above, in the active material for a lithium secondarybattery of the present invention is characterized in that in anLi[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) type ternaryphase diagram, (x, y, z) is represented by values in a range present onor within a line of a heptagon (ABCDEFG) defined by the vertexes; pointA(0.45, 0.55, 0), point B(0.63, 0.37, 0), point C(0.7, 0.25, 0.05),point D(0.67, 0.18, 0.15), point E(0.75, 0, 0.25), point F(0.55, 0,0.45), and point G(0.45, 0.2, 0.35), and that the intensity ratiobetween the diffraction peaks on (003) plane and (104) plane measured byX-ray diffractometry before charge-discharge is I₍₀₀₃₎/I₍₁₀₄₎≧1.56 andat the end of discharge is I₍₀₀₃₎/I₍₁₀₄₎>1.

As shown in FIG. 13 and following Table 1 and Table 2, in a case where(x, y, z) is a value in the above-mentioned range, the dischargecapacity in a potential rang of 4.3 V or lower becomes 177 mAh/g orhigher; however if it is out of the range, only a discharge capacity of176 mAh/g or lower is obtained and in order to obtain an active materialcapable of increasing the discharge capacity, (x, y, z) needs to be inthe specified range.

Further, it is found that within the heptagon ABCDEFG, if (x, y, z) isin a range present on or within a line of a tetragon HIJK defined by thevertexes; point H(0.6, 0.4, 0), point 1(0.67, 0.13, 0.2), point J(0.7,0, 0.3), and point K(0.55, 0.05, 0.4), a particularly high dischargecapacity (198 mAh/g or higher) can be obtained.

Furthermore, with respect to the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ betweenthe diffraction peaks on (003) plane and (104) plane measured by X-raydiffractometry, the following can be assumed.

The active material with a composition formula:Li_(1+(1/3)x)Co_(1−x−y)Ni_((1/2)y)Mn_((2/3)x+(1/2)y) (x+y≦1 and 0≦y) hasa layered structure as shown in FIG. 3 and the Me³⁺ layer is configuredby Li⁺, Co³⁺, Ni²⁺, and Mn⁴⁺. Further, with respect to the activematerial having a layered structure as shown in FIG. 3, if Ni³⁺ iscontaminated in a portion of the Li⁺ layer and Li⁺ is contaminated in aportion of the Ni³⁺ layer, the intensity of I₍₁₀₄₎ is supposed to behigher. Therefore, taking up a representative layered oxide, LiNiO₂(Me³⁺ layer is only Ni³⁺), and assuming that so-called disorder phase(Li_(0.8)Ni_(0.2))[Ni_(0.8)Li_(0.2)]O₂ in which Ni³⁺ is contaminated ina portion of the Li⁺ layer and Li⁺ is contaminated in a portion of theNi³⁺ layer is formed, the X-ray diffraction pattern is simulated bytheoretical calculation and the result is shown in FIG. 4.

As shown in FIG. 4A, LiNiO₂ has a intensity ratio I₍₀₀₃₎/I₍₁₀₄₎=1.10 andthe (003) diffraction peak is sufficiently high; however as shown inFIG. 4B, the intensity ratio of both is considerably changed to beI₍₀₀₃₎/I₍₁₀₄₎≦1 since the disorder phase in which a transition metal(Co, Ni, Mn) is contaminated in the Li layer is formed.

In a conventional active material, it is supposed that such disorderphase is formed to inhibit smooth transfer of Li ion and it affects thereversible capacity.

On the other hand, in the active material of the present invention, itis supposed that since I₍₀₀₃₎/I₍₁₀₄₎≧1.56, formation of the disorderphase is extremely slight and an excellent discharge capacity can beobtained.

The following can be assumed regarding the change of the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ of the diffraction peaks before charge-discharge and thatof the diffraction peaks after charge-discharge after the activematerial production.

Even if the intensity ratio of the diffraction peaks beforecharge-discharge satisfies I₍₀₀₃₎/I₍₁₀₄₎≧1.56, in a case wherecontamination of the transition metals in the Li layer occurs duringdischarge, the diffraction peak on (003) plane becomes broad and at thesame time, the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ becomes significantly smalland in a conventional active material, as shown in FIG. 5, transcriptionof a view described in Non-patent Document 6, the diffraction peak on(104) plane and its intensity may be sometimes reversed.

On the other hand, with respect to the active material of the presentinvention, as shown in Table 1, FIG. 11 and FIG. 12, I₍₀₀₃₎/I₍₁₀₄₎≧1.56before charge-discharge and I₍₀₀₃₎/I₍₁₀₄₎>1 (I₍₀₀₃₎/I₍₁₀₄₎>1.3 inExamples) at the end of discharge and since the intensity of thediffraction peak on (003) plane does not reverse to that of thediffraction peak on (104) plane, it is indicated that contamination ofthe transition metals in the Li layer during charge-discharge does notoccur and accordingly, it is supposed that a stable and high reversiblecapacity can be obtained. At the end of discharge, the intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ may be higher than that before charge-discharge. In a casewhere the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ becomes smaller at the end ofdischarge than before charge-discharge, the change of the intensityratio is preferably slight and it is more preferably within 30% of thatbefore charge-discharge, and it is within 26% in Examples.

Next, a method for producing the active material for a lithium secondarybattery of the present invention will be described.

Basically, the active material for a lithium secondary battery of theinvention is obtained by preparing raw materials containing the metalelements (Li, Mn, Co, Ni) constituting the active material as those inthe composition of the active material (oxide) of interest and calciningthem. However, the amount of a Li raw material is preferable to be inexcess by about 1 to 5% corresponding to the elimination of a portion ofthe Li raw material during calcining.

In order to produce the oxide in the composition of interest, methodsknown are a so-called “solid-state method” involving mixing andcalcining respective salts of Li, Co, Ni, and Mn, and a “coprecipitationmethod” involving previously preparing a coprecipitated precursor inwhich Co, Ni, and Mn are made to be present in each single particle andthen mixing and calcining a Li salt with the precursor. In the synthesisprocess by the “solid-state method”, since Mn is particularly hard touniformly form an solid solution with Co and Ni, it is difficult toobtain a sample of which the respective elements are uniformlydistributed in each single particle. So far, many trials for forming asolid solution of Mn with a portion of Ni or Co by the solid-phasemethod have been performed and reported in documents (e.g.LiNi_(1−x)Mn_(x)O₂), it is easy to obtain a uniform phase in the atomiclevel by selecting the “coprecipitation method”. Therefore, in Examplesdescribed below, the “coprecipitation method” is employed.

In order to produce a coprecipitated precursor, it is extremelyimportant to make a solution from which the coprecipitated precursor isto be obtained inert atmosphere. It is because Mn tends to be oxidizedamong Co, Ni, and Mn and thus production of a coprecipitated hydroxidein which Co, Ni, and Mn are uniformly distributed in divalent state isnot easy and consequently, uniform mixing of Co, Ni, and Mn in theatomic level tends to be insufficient. Particularly, in the compositionrange of the present invention, since the Mn ratio is high as comparedwith the Co and Ni ratios, it is moreover important to make the solutioninert atmosphere. In Examples described below, bubbling of an inert gasis carried out in aqueous solutions to remove dissolved oxygen andfurther a reducing agent is simultaneously dropwise added.

A preparation method of the above-mentioned precursor to be subjected tocalcining is not particularly limited. A Li compound, a Mn compound, aNi compound, and a Co compound may be simply mixed, or a hydroxidecontaining the transition metal elements may be coprecipitated in asolution and then mixed with a Li compound. In order to produce auniform composite oxide, a method of mixing a coprecipitated hydroxideof Mn, Ni, and Co and a Li compound and calcining the mixture ispreferable.

Production of the above-mentioned coprecipitated hydroxide precursor ispreferable to give a compound in which Mn, Ni, and Co are uniformlymixed. However, the precursor is not limited to the hydroxide but otherthan the hydroxide, any compound such as carbonate and citrate may besimilarly employed, if the compounds are hardly soluble salts in whichthe elements are present uniformly in atomic level. Further, acrystallization reaction using a complexing agent may be employed toproduce a precursor with a higher bulk density. At that time, since anactive material with a high density and a small specific surface areacan be obtained by mixing and calcining the precursor with a Li source,the energy density per electrode area can be improved.

Examples of raw materials for the above-mentioned coprecipitatedhydroxide precursor include, as a Mn compound, manganese oxide,manganese carbonate, manganese sulfate, manganese nitrate, and manganeseacetate; as a Ni compound, nickel hydroxide, nickel carbonate, nickelsulfate, nickel nitrate, and nickel acetate; and as a Co compound,cobalt sulfate, cobalt nitrate, and cobalt acetate.

As the raw materials to be used for the production of theabove-mentioned coprecipitated hydroxide precursor, those in any statemay be employed if they can cause precipitation reaction with an aqueousalkaline solution and preferably metal salts with high solubility.

The active material for a lithium secondary battery of the presentinvention can be produced preferably by mixing the coprecipitatedhydroxide precursor with a Li compound and thereafter carrying out heattreatment for the mixture. Use of lithium hydroxide, lithium carbonate,lithium nitrate, or lithium acetate as the Li compound makes it possibleto preferably carry out the production.

In the case of obtaining an active material with a high reversiblecapacity, selection of the calcining temperature is extremely important.

If the calcining temperature is too high, the obtained active materialcorrupts while being accompanied with oxygen releasing reaction and inaddition to the hexagonal main phase, a phase defined as monoclinicLi[Li_(1/3)Mn_(2/3)]O₂ tends to be observed as a separate phase but notas a solid phase and such a material is undesirable since the reversiblecapacity of the active material is considerably decreased. With respectto such a material, impurity peaks are observed near 35° and 45° in theX-ray diffraction pattern. Accordingly, it is important that thecalcining temperature is adjusted lower than the temperature whichaffects the oxygen releasing reaction of the active material. In thecomposition range of the present invention, the oxygen releasingtemperature of the active material is around 1000° C. or higher;however, the oxygen releasing temperature slightly differs based on thecomposition of the active material and therefore it is preferable topreviously confirm the oxygen releasing temperature of the activematerial. Particularly, it is confirmed that the oxygen releasingtemperature of a precursor is shifted to the lower temperature side asthe Co amount contained in a sample is higher and therefore, it shouldbe considered carefully. As a method for confirming the oxygen releasingtemperature of the active material, a mixture of a coprecipitatedprecursor and LiOH.H₂O may be subjected to thermogravimetry (DTA-TGmeasurement) in order to simulate the calcining reaction process;however in this method, platinum employed for a sample chamber of ameasurement instrument may be possibly corroded with an evaporated Licomponent to break the instrument and therefore, a composition of whichcrystallization is promoted to a certain extent by employing a calciningtemperature of about 500° C. is preferable to be subjected tothermogravimetry.

On the other hand, if the calcining temperature is too low, thecrystallization is not carried out sufficiently and the electrodeproperty is also considerably lowered and it is thus not preferable. Thecalcining temperature is required to be at least 800° C. or higher.Sufficient crystallization is important to lower the resistance of grainboundaries and promote smooth lithium ion transfer. A method for carefulevaluation of the crystallization may be visible observation using ascanning electron microscope. When the scanning electron microscopicobservation is carried out for the positive active materials of thepresent invention, at the sample synthesis temperature of 800° C. orlower, there are those made of primary particles in nano-order and someare crystallized to a sub-micron extent by further increasing the samplesynthesis temperature and large primary particles which lead toimprovement of the electrode property can be obtained.

On the other hand, as another factor for showing crystallization, thereis a half width of the X-ray diffraction peak described above. However,merely selection of the synthesis temperature at which the half width ofthe diffraction peak of the main phase is not necessarily adequate toobtain an active material with a high reversible capacity. It is becausethe half width of the diffraction peak is dominated by two factors; oneis the quantity of strain showing the extent of mismatch of the crystallattice and the other is the size of crystallite, which is the minimumdomain and therefore, in order to carefully evaluate the extent ofcrystallinity from the half width, these factors need to be separatelymeasured. The present inventors have confirmed that strains remain inthe lattice in a sample which is synthesized at a temperature up to 800°C. by analysis in detail of the half width of the active material of theinvention and synthesis of the temperature or higher makes it possibleto fairly remove the strains. Further, the size of the crystallitebecomes large in proportional to the increase of the synthesistemperature. Consequently, with respect to the composition of the activematerial of the invention, a desirable discharge capacity is obtained byforming particles sufficiently grown in the crystallite size with scarcestrains in the lattice of the system. More concretely, it is foundpreferable to employ a synthesis temperature (a calcining temperature)at which the strain degree affecting the lattice constant is 1% or lowerand the crystallite size is grown to 150 nm or larger. Although a changedue to expansion and contraction is observed by molding the activematerials into electrodes and carrying out charge-discharge, it ispreferable to keep the crystallite size 130 nm or higher also in thecharge-discharge process as a good effect to be obtained. That is, it ismade at first possible to obtain an active material with a remarkablyhigh reversible capacity by selecting the calcining temperature to be asnear as possible to the oxygen releasing temperature of the activematerial.

As described above, although it is difficult to set a definitelypreferable range of the calcining temperature since it differs dependingon the oxygen releasing temperature of an active material, it ispreferably 900 to 1100° C., more preferably 950 to 1050° C. sinceexcellent properties can be exhibited.

An nonaqueous electrolyte to be used for the lithium secondary batteryof the present invention is not particularly limited and those generallyproposed for use for lithium batteries and the like can be used.Examples of nonaqueous solvents to be used for the nonaqueouselectrolyte can include, but are not limited to, one compound or amixture of two or more of compounds of cyclic carbonic acid esters suchas propylene carbonate, ethylene carbonate, butylene carbonate,chloroethylene carbonate, and vinylene carbonate; cyclic esters such asγ-butyrolactone and γ-valerolactone; chain carbonates such as dimethylcarbonate, diethyl carbonate, and ethylmethyl carbonates; chain esterssuch as methyl formate, methyl acetate, and methyl butyrate;tetrahydrofuran and derivatives thereof, ethers such as 1,3-dioxane,1,4-dioxane, 1,2-dimethoxyethane, 1,4-dibutoxyethane, and methyldiglyme; nitriles such as acetonitrile and benzonitrile; dioxolan andderivatives thereof, and ethylene sulfide, sulfolane, sulfone andderivatives thereof.

Examples of electrolytic salts to be used for the nonaqueous electrolyteinclude inorganic ionic salts containing one of lithium (Li), sodium(Na), and potassium (K) such as LiClO₄, LiBF₄, LiAsF₆, LiPF₆, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, NaClO₄, NaI, NaSCN, NaBr, KClO₄, andKSCN; and organic ionic salts such as LiCF₃SO₃, LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂), LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃,(CH₃)₄NBF₄, (CH₃)₄NBr, (C₂H₅)₄NClO₄, (C₂H₅)₄NI, (C₃H₇)₄NBr,(n-C₄H₉)₄NClO₄, (n-C₄H₉)₄NI, (C₂H₅)₄N-maleate, (C₂H₅)₄N-benzoate,(C₂H₅)₄N-phthalate, lithium stearylsulfonate, lithium octylsulfonate,and lithium dodecylbenzenesulfonate and these ionic compounds may beused alone or in combination of two or more of them.

Further, if a lithium salt having a perfluoroalkyl group such as LiBF₄and LiN(C₂F₅SO₂)₂ is added to be used, the viscosity of the electrolytecan be lowered and therefore the low temperature properties can befurther improved and self-discharge can be suppressed and therefore, itis preferable.

Further, a normal temperature molten salt or ionic liquid may be used asthe nonaqueous electrolyte.

The concentration of the electrolytic salt in the nonaqueous electrolyteis preferably 0.1 mold to 5 mold and more preferably 0.5 mol/l to 2mol/l to reliably obtain a nonaqueous electrolyte battery having highbattery properties.

A negative electrode material is not particularly limited and may be anyif it can precipitate or absorb lithium ions. Examples thereof include atitanium type materials such as lithium titanate having a spinel typecrystal structure typified by Li[Li_(1/3)Ti_(5/3)]O₄; alloy type lithiummetal and lithium alloys of Si and Sb and Sn (lithium metal-containingalloy such as lithium-silicon, lithium-aluminum, lithium-lead,lithium-tin, lithium-aluminum-tin, lithium-gallium, and Wood' alloy),lithium composite oxide (lithium-titanium), silicon oxide as wells asalloys capable of absorbing and releasing lithium, and carbon materials(e.g. graphite, hard carbon, low temperature calcined carbon, amorphouscarbon).

A powder of the positive active material and a powder of the negativeactive material preferably have an average particle size of 100 μm orsmaller. Particularly, the powder of the positive active material isdesirable to be 10 μm or smaller in order to improve the high outputperformance of the nonaqueous electrolyte battery. In order to obtain apowder in a prescribed shape, a pulverizer or a classifier may be used.For example, usable are mortars, ball mills, sand mills, vibration ballmills, planet ball mills, jet mills, counter jet mills, swirling currenttype jet mill, and sieves. At the time of pulverization, wetpulverization in co-presence of water or an organic solvent such ashexane can also be employed. A classification method is not particularlylimited and sieves, pneumatic classifiers and the like may be employedin both dry and wet manner if necessary.

The positive active material and the negative active material, which aremain constituent components of a positive electrode and a negativeelectrode are described in detail, and the positive electrode and thenegative electrode may contain an electric conductive agent, a binder, athickener, a filler and the like as other constituent components besidesthe above-mentioned main constituent components.

The electric conductive agent is not particularly limited if it is anelectron conductive material causing no adverse effect on the batteryperformance and it may be, in general, electric conductive materialssuch as natural graphite (scaly graphite, flaky graphite, earthygraphite), artificial graphite, carbon black, acetylene black, Ketjenblack, carbon whisker, carbon fibers, powders of metals (copper, nickel,aluminum, silver, gold, etc.), metal fibers and electric conductiveceramic materials, and one or a mixture of these materials may be used.

As an electric conductive agent among them, acetylene black ispreferable form the viewpoints of electron conductivity and coatability.The addition amount of the electric conductive agent is preferably 0.1%by weight to 50% by weight and particularly preferably 0.5% by weight to30% by weight based on the total weight of the positive electrode or thenegative electrode. Particularly, if acetylene black is used while beingpulverized into ultrafine particles of 0.1 to 0.5 μm, the carbon amountto be needed can be saved and therefore it is preferable. A mixingmethod of them may be physical mixing and ideally, it is uniform mixing.For this reason, powder mixers such as V-shaped mixers, S-shaped mixers,attriters, ball mills, and planet ball mills may be used to carry outdry or wet mixing.

As the binder, in general, thermoplastic resins such aspolytetrafluoroethylene (PTFE), poly(vinylidene fluoride) (PVDF),polyethylene, and polypropylene; and polymers having rubber elasticitysuch as ethylene-propylene-diene-terpolymers (EPDM), sulfonated EPDM,styrene-butadiene rubber (SBR), fluoro rubber can be used alone or incombination of two or more of them. The addition amount of the binder ispreferably 1 to 50% by weight and particularly preferably 2 to 30% byweight based on the total weight of the positive electrode or thenegative electrode.

The filler is not particularly limited if it is a material causing noadverse effect on the battery performance. In general, usable may beolefin type polymers such as polypropylene and polyethylene; amorphoussilica, alumina, zeolite, glass, carbon and the like. The additionamount of the filler is preferably 30% by weight or less based on thetotal weight of the positive electrode or the negative electrode.

The positive electrode and the negative electrode can be preferablyproduced by mixing the main constituent components (the positive activematerial in the positive electrode and the negative active material inthe negative electrode) and other materials to obtain composites, thenmixing the composites with an organic solvent such asN-methylpyrrolidone, toluene, or the like, applying the obtained mixedsolutions onto current collectors described below or bonding thesolution with pressure, and carrying out heat treatment at a temperatureof about 50° C. or 250° C. for about 2 hours. The application method ispreferably carried out to give an arbitrary thickness and an arbitraryshape by using means such as roller coating such as applicator rolls,screen coating, doctor blade coating manner, spin coating, and barcoaters; however it is not limited to thereto.

As a separator, porous membranes and nonwoven fabrics having excellenthigh rate discharge performance may be used preferably alone or incombination. Examples of materials constituting a separator for anonaqueous electrolyte battery can include polyolefin type resinstypified by polyethylene and polypropylene; polyester type resinstypified by poly(ethylene terephthalate) and poly(butyleneterephthalate); poly(vinylidene fluoride), vinylidenefluoride-hexafluoropropylene copolymers, vinylidenefluoride-perfluorovinyl ether copolymers, vinylidenefluoride-tetrafluoroethylene copolymers, vinylidenefluoride-trifluoroethylene copolymers, vinylidenefluoride-fluoroethylene copolymers, vinylidenefluoride-hexafluoroacetone copolymers, vinylidene fluoride-ethylenecopolymers, vinylidene fluoride-propylene copolymers, vinylidenefluoride-trifluoropropylene copolymers, vinylidenefluoride-tetrafluoroethylene-hexafluoropropylene copolymers, andvinylidene fluoride-ethylene-tetrafluoroethylene copolymers.

The porosity of the separator is preferable 98% by volume or less fromthe viewpoint of strength. Further, from the viewpoint ofcharge-discharge property, the porosity is preferably 20% by volume orhigher.

The separator may be a polymer gel configured by, for example, a polymerof acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate,vinyl acetate, vinylpyrrolidone, and poly(vinylidene fluoride) and anelectrolyte. If the nonaqueous electrolyte is used in a gel state asdescribed above, it is preferable since it is effective to preventliquid leakage.

Further, in the separator, the above-mentioned porous membranes ornonwoven fabrics are used in combination with the polymer gel, it ispreferable since the electrolyte retention property is improved. Thatis, a film is obtained by coating the surface and fine pore wallsurfaces of a polyethylene fine porous membrane with a solvophilicpolymer in a thickness of several μm or less and making the fine poresof the film keep the electrolyte, so that the solvophilic polymer can beformed into gel.

Examples of the solvophilic polymer include poly(vinylidene fluoride)and also polymers crosslinked by acrylate monomers having ethylene oxidegroups or ester groups, epoxy monomers, and monomers having isocyanatogroups. Crosslinking reaction of the monomers may be carried out byheating or using ultraviolet rays (UV) with a radical initiator incombination or using activation beam such as electron beam.

The configuration of the lithium secondary battery is not particularlylimited and examples thereof include cylindrical batteries, prismaticbatteries, and flat type batteries including the positive electrode,negative electrode, and roll type separator.

EXAMPLES

The compositions of positive active materials used for lithium secondarybatteries of Examples and Comparative Examples are shown in Table 1. Thecompositions of Examples 1 to 44 satisfy the composition formula:Li_(1+(1/3)x)Co_(1−x−y)Ni_((1/2)y)Mn_((2/3)x+(1/2)y) (x+y≦1, 0≦y,1−x−y=z) and also satisfy the range disclosed in claim 1: althoughComparative Examples 1 to 40 satisfy the above-mentioned compositionformula, the value of (x, y, z) is out of the range disclosed in claim1: and Comparative Examples 41 to 43 do not satisfy even the compositionformula. That is, in FIG. 1, the compositions of Examples 1 to 44 arepresent on or within a line of a heptagon ABCDEFG and the compositionsof Comparative Examples 1 to 40 are present outside of the heptagonABCDEFG.

Example 1

An aqueous mixed solution was produced by dissolving manganese sulfatepentahydrate, nickel sulfate hexahydrate, and cobalt sulfateheptahydrate at a ratio of 0.25:0.17:0.45 of the respective elements Co,Ni, and Mn in ion-exchanged water. At that time, the total concentrationwas adjusted to 0.667 M and the volume to 180 ml. Next, 600 ml of ionexchanged water was made available in a 1 L beaker and using a hot bathto keep the temperature at 50° C., 8N NaOH was dropwise added to adjustthe pH 11.5. In such a state, bubbling with Ar gas was carried out for30 min to sufficiently remove dissolved oxygen in the solution. Thecontent in the beaker was stirred at 700 rpm, the preparedsulfates-mixed aqueous solution was added dropwise at a speed of 3ml/min. During the time, the temperature was kept constant by the hotbath and pH was kept constant by intermittently adding 8 N NaOHdropwise. Simultaneously, 50 ml of an aqueous 2.0 M hydrazine solutionas a reducing agent was added dropwise at a speed of 0.83 ml/min. On thecompletion of the dropwise addition of both, the stirring was stoppedand the solution was kept still for 12 hours or longer to sufficientlygrow particles of a coprecipitated hydroxide.

Herein, in the above-mentioned procedure, if the dropwise addition speedof each solution was too high, it became impossible to obtain a uniformprecursor in atomic level. For example, in a case where the dropwiseaddition speed was increased 10 times as fast as that described above,the fact that the element distribution in the precursor was apparentlyununiform was made clear from the results of EPMA measurement. Further,it was also confirmed that in a case where an active material wassynthesize using such ununiform precursor, the distribution of elementsafter calcining also became ununiform and it resulted in impossibilityof exhibiting sufficient electrode properties. In this connection, inthe case of using LiOH.H₂O, Co(OH)₂, Ni(OH)₂, and MnOOH as raw materialpowders in a solid-phase method, further ununiformity was proved by theresults of EPMA measurement.

Next, the coprecipitation product was taken out by suction filtrationand dried at 100° C. in atmospheric air and normal pressure in an oven.After drying, in order to adjust particle diameter, the product waspulverized for several minutes by a mortar with a diameter of about 120mmφ to obtain a dried powder.

By X-ray diffractometry, the dried powder was confirmed to have aβ-Ni(OH)₂ type single phase. Further, Co, Ni, and Mn were confirmed tobe present uniformly by EPMA measurement.

A lithium hydroxide monohydrate salt powder (LiOH.H₂O) was weighed tomake the Li amount to the transition metals (Ni+Mn+Co) satisfy thecomposition formula of Example 1 in Table 1 and mixed to obtain a mixedpowder.

Next, the mixed powder was pellet-molded at a pressure of 6 MPa. Theamount of the precursor powder supplied to the pellet molding wasdetermined by calculation for controlling the mass as a product aftersynthesis to be 3 g. As a result, the pellets after molding had adiameter of 25 mmφ, thickness about 10 to 12 mm. The pellets were put onan alumina boat with a whole length of about 100 mm, and then set in abox type electric furnace and calcined at 1000° C. for 12 hours inatmospheric air under normal pressure. The inside size of the boxy typeelectric furnace was 10 cm height, 20 cm width, and 30 cm depth andheating wires were set at 20 cm intervals in the width direction. Aftercalcination, a switch of the heater was turned off and the alumina boatwas left in the furnace as it was to carry out spontaneous cooling. As aresult, the temperature of the furnace was lowered to about 200° C.after 5 hours; however the rate of the temperature decrease thereafterwas slightly slow. After overnight, the temperature of the furnace wasconfirmed to be 100° C. or lower and thereafter the pellets were takenout and pulverized to make the particle diameter uniform by using amortar.

The crystal structure of the obtained active material was confirmed tocontain an α-NaFeO₂ type hexagonal structure as a main phase accordingto the results of powder X-ray diffractometry using a Cu(Kα) radiationand at the same time was observed to have a diffraction peak around 20to 30° which is obtained Partially for a monoclinicLi[Li_(1/3)Mn_(2/3)]O₂. FIG. 6 shows the X-ray diffraction pattern ofthe active material (AT06) of Example 1. The intensity ratioI₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003) plane and (104)plane before charge-discharge was 1.69. Further, the count number of thediffraction peak at 21° which is observed for the monoclinicLi[Li_(1/3)Mn_(2/3)]O₂ was 7 in a case where the count number of thepeaks near 18° showing the maximum intensity was assumed to be 100.

Further, XAFS measurement was carried out for valence evaluation of thetransition metal elements. When the spectrometric analysis was carriedout for the XANES region, it was confirmed that Co³⁺, Ni²⁺, and Mn⁴⁺were in electron state. The results of XANES measurement are shown inFIG. 7.

Examples 2 to 44

The active materials of the present invention were synthesize in thesame manner as Example 1, except that the compositions of the transitionmetal elements contained in the coprecipitated hydroxide precursors andthe amount of lithium hydroxide to be mixed was changed according to thecomposition formulas shown in Examples 2 to 44 shown in Table 1.

As a result of X-ray diffractometry, similarly to Example 1, theα-NaFeO₂ type hexagonal structure was confirmed to be a main phase andalso a diffraction peak around 20 to 300 which is obtained partially formonoclinic Li[Li_(1/3)Mn_(2/3)]O₂ was observed. Further, as shown inTable 1, the intensity ratio I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peakson (003) plane and (104) plane before charge-discharge was all 1.56 orhigher.

Comparative Examples 1 to 40

The active materials of Comparative Examples were synthesize in the samemanner as Example 1, except that the compositions of the transitionmetal elements contained in the coprecipitated hydroxide precursors andthe amount of lithium hydroxide to be mixed was changed according to thecomposition formulas shown in Comparative Examples 1 to 40 shown inTable 1.

FIG. 8 shows the X-ray diffraction pattern of the active material ofComparative Example 3 (AT09) as representative. With respect toComparative Examples 12 to 18 and 33 to 40 in which the x value is ⅔ ormore, similarly to Example 1, the α-NaFeO₂ type hexagonal structure wasconfirmed to be a main phase and also a diffraction peak around 20 to30° which is obtained partially for monoclinic Li[Li_(1/3)Mn_(2/3)]O₂was observed. However, with respect to Comparative Examples 1 to 11 and19 to 32 in which the x value is ⅓ or less, although the α-NaFeO₂ typehexagonal structure was confirmed, no diffraction peak observed formonoclinic Li[Li_(1/3)Mn_(2/3)]O₂ was clearly observed as long as thepeak height of the maximum intensity in the X-ray diffraction patternwas enlarged to the full scale. Further, as shown in Table 1, theintensity ratios I₍₀₀₃₎/I₍₁₀₄₎ between the diffraction peaks on (003)plane and (104) plane of the active materials before charge-dischargewere 1.43 or higher, however some were 1.56 or lower.

Comparative Examples 41 and 42

The active materials of Comparative Examples 41 and 42 were synthesizein the same manner as Example 1, except that the compositions of thetransition metal elements contained in the coprecipitated hydroxideprecursors and the amount of lithium hydroxide to be mixed was changedaccording to the composition formula: LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂.

Herein, Comparative Examples 41 and 42 were different from each other inthe set values of the charging voltage in the test condition describedbelow (Comparative Example 41: 4.6 V, Comparative Example 42: 4.3 V) andwere identical with each other as the active material.

Comparative Example 43

The active material of Comparative Example 43 was synthesize in the samemanner as Example 1, except that a powder obtained by mixing respectivepowders of LiOH.H₂O, Co(OH)₂, Ni(OH)₂, and MnOOH at element ratio ofLi:Co:Ni:Co=1:0.33:0.33:0.33 was used in place of the coprecipitatedhydroxide precursor powder. The X-ray diffraction pattern could not bediscriminated from those of Comparative Examples 1 and 42. However, fromthe results of EPMA observation, Co, Ni, and Mn were not uniformlydistributed in the material.

(Production and Evaluation of Lithium Secondary Batteries)

Using the respective active materials of Examples 1 to 44 andComparative Examples 1 to 43 as a positive active material for a lithiumsecondary battery, lithium secondary batteries were produced in thefollowing procedure and the battery properties were evaluated.

Each coating solution was prepared by mixing each active material,acetylene black (AB), and poly(vinylidene fluoride) (PVdF) at a weightratio of 85:8:7 and adding N-methylpyrrolidone as a dispersion mediumand mixing and dispersing these compounds. As PVdF, a liquid in whichsolid matter was dissolved and dispersed was used and solid matterweight conversion was carried out. The coating solution was applied toan aluminum foil current collector with a thickness of 20 μm to produceeach positive electrode plate. In all batteries, the electrode weightand thickness were standardized to make the same test conditions for allof the batteries.

As a counter electrode, lithium metal was used for a negative electrodeto observe the behavior of each positive electrode alone. The lithiummetal was closely attached to a nickel foil current collector. However,it was prepared in such a manner that the capacity of each lithiumsecondary battery was controlled sufficiently by the positive electrode.

As an electrolyte, a solution was used which was obtained by dissolvingLiPF₆ in a solvent mixture of EC/EMC/DMC at a ratio of 6:7:7 by volumeto give a concentration of 1 mol/L. As a separator, a finely porous filmmade of polypropylene was used which was provided with improvedelectrolyte retention property by surface modification withpolyacrylate. Further, an electrode obtained by sticking a lithium metalfoil to a nickel plate was used as a reference electrode. As an outercasing, a metal resin composite film was used which was made ofpoly(ethylene terephthalate) (15 μm)/aluminum foil (50μm)/metal-adhesive polypropylene film (50 μm) and the electrodes werehoused in such a manner that the opened terminal parts of the positiveelectrode terminal, negative electrode terminal, and reference electrodeterminal were exposed to the outside and fusion-melting margins wherethe inner surfaces of the metal resin composite films were mutuallyencountered were tightly sealed except the portion where an injectionhole was to be formed.

Each lithium secondary battery produced in the above-mentioned mannerwas subjected to the initial charge-discharge process of 5 cycles at 20°C. The voltage control was all carried out for the positive electrodepotential. Charge was carried out at constant current and constantvoltage charge for 0.1 ItA and 4.5 V and the condition of ending thecharge was set to be the time point when the electric current value wasdecreased to ⅙. Discharge was carried out at constant current for 0.1ItA and 2.0 V at the end. In all cycles, a 30 minute-rest was set aftercharge and after discharge. The behavior of the first two cycles in theinitial charge-discharge process is shown in FIG. 9. FIG. 9A and FIG. 9Bcorrespond to Example 6 (AT17) and Comparative Example 4 (AT11),respectively.

Successively, a charge-discharge cycle test was carried out. The voltagecontrol was carried out all for the positive electrode potential. Theconditions of the charge-discharge cycle test were the same as those inthe above-mentioned initial charge-discharge process, except that thecharge voltage was set to 4.3 V (vs. Li/Li⁺) (4.6 V only for ComparativeExample 41). In all cycles, a 30 minute-pause was set after charge andafter discharge. The discharge electric quantity at 5th cycle wasrecorded as “discharge capacity (mAh/g)”. FIG. 10 shows a representativecharge-discharge curve of the 5th cycle in this charge-discharge cycletest.

Further, percentage of the discharge electric quantity at 10th cycle inthe charge-discharge cycle test to the above-mentioned “dischargecapacity (mAh/g)” was measured and defined as “capacity retention ratio(%)”.

Similarly to the measurement before charge-discharge, aftercharge-discharge, the active materials of Examples 1 to 44 andComparative Examples 1 to 40 were subjected to powder X-raydiffractometry using a Cu(Kα) radiation. Charge was constant current andconstant voltage charge at 0.1 ItA current and 4.5 V voltage and theending of the charge was set to be the time point when the electriccurrent value was decreased to ⅙. Thereafter, charging was carried outto 4.3 V (vs. Li/Li⁺) and then constant current discharge at 0.1 ItAcurrent was carried out and the time point when the voltage at the endbecame 2.0 V was defined as the end of the discharge. The X-raydiffraction patterns of the active material of Example 7 (AT18) and theactive material of Example 16 (AT33) before charge-discharge(synthesized samples), at end of charge, and at end of discharge areshown respectively in FIG. 11 and FIG. 12.

With respect to the active materials of Examples 1 to 44 and ComparativeExamples 1 to 40, the results of the battery test (excluding thecapacity retention ratio) are show in Table 1 and Table 2. FIG. 13 showsthe values of the discharge capacities by plotting them in aLi[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) type ternaryphase diagram.

TABLE 1 Before At end of Discharge Experiment charge-discharge chargecapacity Li[Li_(1/3)Mn_(2/3)]O₂ LiNi_(0.5)Mn_(0.5)O₂ LiCoO₂ Example No.No. I_((0.03))/I₍₁₀₄₎ I_((0.03))/I₍₁₀₄₎ (mAh/g) (x) (y) (z) Example 1AT06 1.69 1.43 189 0.50 0.25 0.25 Example 2 AT14 1.77 1.68 186 0.45 0.200.35 Example 3 AT15 1.79 1.44 182 0.45 0.40 0.15 Example 4 AT16 1.651.39 180 0.45 0.50 0.05 Example 5 AT22 1.84 1.63 220 0.60 0.10 0.30Example 6 AT17 1.77 1.67 225 0.60 0.20 0.20 Example 7 AT18 1.68 1.61 2240.60 0.30 0.10 Example 8 AT19 1.61 1.43 219 0.60 0.40 0.00 Example 9AT25 2.00 1.42 177 0.70 0.10 0.20 Example 10 AT27 1.64 1.60 180 0.670.23 0.10 Example 11 AT28 1.68 1.53 223 0.67 0.13 0.20 Example 12 AT291.78 1.50 207 0.67 0.03 0.30 Example 13 AT30 1.63 1.50 187 0.67 0.180.15 Example 14 AT31 1.62 1.56 185 0.67 0.00 0.33 Example 15 AT32 1.661.31 187 0.70 0.05 0.25 Example 16 AT33 1.78 1.31 198 0.70 0.00 0.30Example 17 AT51 1.94 2.08 179 0.50 0.10 0.40 Example 18 AT53 2.07 2.18183 0.60 0.00 0.40 Example 19 AT54 1.57 1.47 185 0.50 0.50 0.00 Example20 AT55 1.91 1.75 185 0.50 0.40 0.10 Example 21 AT56 2.06 1.79 185 0.500.30 0.20 Example 22 AT57 2.13 2.40 185 0.50 0.20 0.30 Example 23 AT581.61 1.90 190 0.63 0.37 0.00 Example 24 AT59 1.66 1.46 191 0.63 0.320.05 Example 25 AT60 1.85 2.10 206 0.63 0.27 0.10 Example 26 AT61 1.931.81 200 0.63 0.22 0.15 Example 27 AT62 1.94 1.78 206 0.63 0.17 0.20Example 28 AT63 2.17 1.79 200 0.63 0.07 0.30 Example 29 AT64 2.19 2.21203 0.60 0.05 0.35 Example 30 AT65 2.16 1.61 200 0.67 0.08 0.25 Example31 AT66 1.56 1.54 182 0.45 0.55 0.00 Example 32 AT67 1.83 1.99 177 0.450.30 0.25 Example 33 AT68 1.96 2.22 179 0.45 0.10 0.45 Example 34 AT692.11 2.27 186 0.50 0.15 0.35 Example 35 AT70 2.17 2.35 197 0.55 0.100.35 Example 36 AT71 2.22 2.06 199 0.55 0.05 0.40 Example 37 AT72 2.152.08 189 0.75 0.05 0.20 Example 38 AT73 1.81 1.59 180 0.75 0.00 0.25Example 39 AT75 1.87 2.08 178 0.55 0.35 0.10 Example 40 AT76 2.02 1.97185 0.55 0.25 0.20 Example 41 AT77 2.25 2.28 188 0.55 0.15 0.30 Example42 AT78 2.08 2.44 183 0.55 0.00 0.45 Example 43 AT79 1.72 1.77 182 0.670.28 0.05 Example 44 AT81 1.67 1.70 190 0.70 0.25 0.05

TABLE 2 Before Discharge Experiment charge-discharge capacityLi[Li_(1/3)Mn_(2/3)]O₂ LiNi_(0.5)Mn_(0.5)O₂ LiCoO₂ Example No. No.I_((0.03))/I₍₁₀₄₎ (mAh/g) (x) (y) (z) Comparative AT04 1.65 158 0.330.33 0.33 Example 1 Comparative AT05 1.54 149 0.25 0.50 0.25 Example 2Comparative AT09 1.56 148 0.15 0.60 0.25 Example 3 Comparative AT11 1.52152 0.30 0.40 0.30 Example 4 Comparative AT20 2.07 143 0.15 0.20 0.65Example 5 Comparative AT08 1.75 145 0.15 0.40 0.45 Example 6 ComparativeAT10 1.89 145 0.15 0.80 0.05 Example 7 Comparative AT21 1.95 152 0.300.20 0.50 Example 8 Comparative AT12 1.83 154 0.30 0.60 0.10 Example 9Comparative AT13 1.60 154 0.30 0.70 0.00 Example 10 Comparative AT071.76 145 0.25 0.25 0.50 Example 11 Comparative AT23 1.72 176 0.70 0.200.1n Example 12 Comparative AT24 1.70 147 0.80 0.10 0.10 Example 13Comparative AT26 1.43 175 0.67 0.33 0.00 Example 14 Comparative AT341.75 166 0.80 0.05 0.15 Example 15 Comparative AT35 1.84 171 0.80 0.000.20 Example 16 Comparative AT36 1.61 164 0.90 0.05 0.05 Example 17Comparative AT37 1.54 173 0.90 0.00 0.10 Example 18 Comparative AT381.61 172 0.33 0.60 0.07 Example 19 Comparative AT39 1.63 170 0.33 0.470.20 Example 20 Comparative AT40 1.96 157 0.33 0.20 0.47 Example 21Comparative AT41 2.27 156 0.33 0.07 0.60 Example 22 Comparative AT422.62 141 0.30 0.10 0.60 Example 23 Comparative AT43 2.12 130 0.30 0.000.70 Example 24 Comparative AT44 2.50 159 0.40 0.10 0.50 Example 25Comparative AT45 2.46 156 0.40 0.00 0.60 Example 26 Comparative AT461.55 167 0.40 0.60 0.00 Example 27 Comparative AT47 1.72 162 0.40 0.500.10 Example 28 Comparative AT48 1.69 158 0.40 0.40 0.20 Example 29Comparative AT49 1.73 170 0.40 0.30 0.30 Example 30 Comparative AT502.33 152 0.40 0.20 0.40 Example 31 Comparative AT52 2.09 173 0.50 0.000.50 Example 32 Comparative AT80 1.61 139 0.70. 0.30 0.00 Example 33Comparative AT82 2.02 172 0.70 0.15 0.15 Example 34 Comparative AT831.67 147 0.75 0.25 0.00 Example 35 Comparative AT84 1.76 172 0.75 0.200.05 Example 36 Comparative AT85 2.12 160 0.75 0.15 0.10 Example 37Comparative AT86 2.11 155 0.75 0.10 0.15 Example 38 Comparative AT871.67 147 0.80 0.20 0.00 Example 39 Comparative AT88 1.71 149 0.80 0.150.05 Example 40

As being understood from Table 1, Table 2, and FIG. 13, use of theactive materials of Examples 1 to 44 having values of (x, y, z) in arange present on or within a line of a heptagon (ABCDEFG) defined by thevertexes; point A(0.45, 0.55, 0: corresponding to the composition ofAT66), point B(0.63, 0.37, 0: corresponding to the composition of AT58),point C(0.7, 0.25, 0.05: corresponding to the composition of AT81),point D(0.67, 0.18, 0.15: corresponding to the composition of AT30),point E(0.75, 0, 0.25: corresponding to the composition of AT73), pointF(0.55, 0, 0.45: corresponding to the composition of AT78), and pointG(0.45, 0.2, 0.35: corresponding to the composition of AT14) made itpossible to obtain lithium secondary batteries with discharge capacitiesas high as 177 mAh/g or higher in a potential region of 4.3 V or lower.Those using the active materials of Comparative Examples 1 to 40 out ofthe above-mentioned range had discharge capacities of 176 mAh/g orlower. Especially, in the case of using active materials havingparticularly specified values (x, y, z) in a range present on or withina line of a tetragon (HIJK) defined by the vertexes; point H(0.6, 0.4,0: corresponding to the composition of AT19), point 1(0.67, 0.13, 0.2:corresponding to the composition of AT28), point J(0.7, 0, 0.3;corresponding to the composition of AT33), and point K(0.55, 0.05, 0.4:corresponding to the composition of AT71) (Examples 5 to 8, Examples 11,12, 16, 25 to 30, and 36), lithium secondary batteries with dischargecapacities as high as 198 mAh/g or higher in a potential region of 4.3 Vor lower could be obtained.

Further, with respect to LiCo_(1/3)Ni_(1/3)Mn_(1/3)O₂, in a case wherethe discharge potential was adjusted to 4.6 V as in Comparative Example41, the discharge capacity was 181 mAh/g; however, in a case where thedischarge potential was adjusted to 4.3 V as in Comparative Example 42,the discharge capacity was 149 mAh/g and therefore, the value of thedischarge capacity of the active material of the present inventionexceeds that of Li[Co_(1−2x)Ni_(x)Mn_(x)]O₂ (0≦x≦½) or LiNiO₂ type,which is regarded as representative of high capacity.

As shown in Table 1, the active material of the present invention hadthe intensity ratio of the diffraction peaks satisfyingI₍₀₀₃₎/I₍₁₀₄₎≧1.56 before charge-discharge and I₍₀₀₃₎/I₍₁₀₄₎>1.3exceeding I₍₀₀₃₎/I₍₁₀₄₎>1 at the end of discharge and moreover, sincethe change of the intensity ratio at the end of discharge was within 26%of that before charge-discharge, it is indicated that no contaminationof the transition metals to the Li layer during charge-discharge wasgenerated and at this point, the active material is apparentlydistinguished from the conventionalLi[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) type activematerial.

Furthermore, with respect to the capacity retention, the lithiumsecondary batteries using the active materials of Examples 1 to 44 kept100%, whereas the lithium secondary batteries using the active materialsof Comparative Examples 41, 42, and 43 kept only 89%, 98%, and 80%,respectively, and therefore, the lithium secondary battery of thepresent invention is found excellent also in the charge/discharge cycleperformance.

1. A process for producing a lithium secondary battery which employscharging method where a positive electrode upon charging has a maximumachieved potential of 4.3 V (vs. Li/Li+) or lower, comprising: chargingthe lithium secondary battery to reach at least a region with relativelyflat fluctuation of potential appearing in a positive electrodepotential region exceeding 4.3 V (vs. Li/Li⁺) and 4.8V (vs. Li/Li⁺) orlower, wherein the lithium secondary battery includes an active materialcomprising a solid solution of a lithium transition metal compositeoxide having an α-NaFeO₂ type crystal structure; the composition ratioof Li, Co, Ni, and Mn contained in the solid solution satisfiesLi_(1+1/3x)Co_(1−x−y)Ni_(y/2)Mn_(2x/3+y/2)(x+y≦1, 0≦y and 1−x−y=z); inan Li[Li_(1/3)Mn_(2/3)]O₂(x)-LiNi_(1/2)Mn_(1/2)O₂(y)-LiCoO₂(z) typeternary phase diagram, (x, y, z) is represented by values in a rangepresent on or within a line of a heptagon (ABCDEFG) defined by thevertexes; point A(0.45, 0.55, 0), point B(0.63, 0.37, 0), point C(0.7,0.25, 0.05), point D(0.67, 0.18, 0.15), point E(0.75, 0, 0.25), pointF(0.55, 0, 0.45), and point G(0.45, 0.2, 0.35).
 2. The process forproducing a lithium secondary battery according to claim 1, wherein thecharging to reach at least a region with relatively flat fluctuation ofpotential appearing in a positive electrode potential region exceeding4.3 V (vs. Li/Li⁺) and 4.8V (vs. Li/Li⁺) or lower is the charging in aninitial charge-discharge process.
 3. The process for producing a lithiumsecondary battery according to claim 1, wherein a dischargeable electricquantity in a potential region of 4.3V (vs. Li/Li⁺) or lower is 177mAh/g or higher.
 4. The process for producing a lithium secondarybattery according to claim 1, wherein (x, y, z) is represented by valuesin a range present on or within a line of a tetragon (HIJK) defined bythe vertexes; point H(0.6, 0.4, 0), point I (0.67, 0.13, 0.2), point J(0.7, 0, 0.3), and point K (0.55, 0.05, 0.4).
 5. The process forproducing a lithium secondary battery according to claim 4, wherein adischargeable electric quantity in a potential region of 4.3V (vs.Li/Li⁺) or lower is 200 mAh/g or higher.
 6. The process for producing alithium secondary battery according to claim 1, wherein the solidsolution of lithium transition metal composite oxide has a diffractionpeak observed near 20 to 30° in X-ray diffractometry using CuK aradiation for the monoclinic Li[Li_(1/3)Mn_(2/3)]O₂ beforecharge-discharge and the intensity of the diffraction peak is about 4 to7% of the intensity of the diffraction peak of the (003) plane.
 7. Theprocess for producing a lithium secondary battery according to claim 1,wherein the intensity ratio between the diffraction peaks on (003) planeand (104) plane measured by X-ray diffractometry before charge-dischargeis I₍₀₀₃₎/I₍₁₀₄₎ is ≧1.56 and at the end of discharge is >1.
 8. Theprocess for producing a lithium secondary battery according to claim 1,wherein the solid solution of lithium transition metal composite oxidehas valances of respective metal elements as Li⁺, Co³⁺, Ni²⁺, and Mn⁴⁺.9. The process for producing a lithium secondary battery according toclaim 1, further comprising preparing the solid solution of lithiumtransition metal composite oxide by a coprecipitation method.
 10. Theprocess for producing a lithium secondary battery according to claim 9,wherein the process of preparing the solid solution of a lithiumtransition metal composite oxide, comprising: coprecipitating a Cocompound, a Mn compound, and a Ni compound in a solution to make aprecursor; and mixing and calcining the precursor and a Li compound.